Because there was no technique available in the past that could make oceanographers able to see inside the water, they fell back to (relied on) blind groping (that means feel about with hands due to lack of visibility) deep inside the marine water.The sentence hints that we already have the technology now that equips us to see inside the water.

Because there is no light deep in the ocean it is very difficult to study the life, chemistry and geology of the ocean at extreme depths. So until recently (now there must be better technology) oceangraphers had to search or feel about like blind people in order to study the deepest parts of the sea.

oceanographers (people who study the ocean) could not see what was happening deep in the ocean, for a long time . Their only method was blind groping.... feeling things underwater without actually seeing what they were touching. they relied on that method... which means they trusted that method, and used it to continue their research and observations. Recently, they found a way, or they invented the technology, to look at the underwater world, to 'see' what is down there.. so they are not 'groping anymore', which is a good thing for all of us.